Lesson 3
Laboratory work in TDDI04
Assignment 3
» Lesson purposes
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»Viacheslav Izosimov
»2009-02-23
»viaiz@ida.liu.se
Threads
Pintos organization
User programs
» Smallest execution unit
» Own stack
» User-thread run in a process address
User memory
User stack
Threads
Scheduler
Short repetition of Processes and Threads
Virtual memory layout in PINTOS
Protection of system calls
Exercises about virtual memory and
protection/security
space (user-mode)
Kernel stack
Synchronization
User-exception handler
primitives
Memory management
Interrupt handler
File system
Timer
Threads
» Kernel-thread run OS-code in physical
address space (kernel-mode)
» Share memory with all threads in same
address space
Processes
A system contains several threads running in parallel
Thread is a “basic unit of CPU utilization”
§Thread ID
§A program counter
§A register set
§A stack
Thread 1
Thread 2
» Program in execution
» Own stack
» Define own independent address space
» Powered by kernel thread(s)
» Can contain several user threads
» Run in user mode
Thread execution sequence
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Process and thread relations
System call
Process memory, Logic address space
User
thread
User
thread
Process
User
thread
User
Mode
Process
resources controlled by kernel
» A ”system call” is the request for this
service, and handling thereof
Process
Process
» Process run in user mode
» Process need access to shared
System call interface
Kernel
thread
Kernel
thread
Kernel
thread
Kernel
thread
Kernel
Mode
Physical memory address space
Create example
» bool create (const char *file, unsigned
initial_size)
» Example: create(“file.txt”, 1000);
How to get them?
Answer: f->esp
Hint:
… note that, in order to get
How to return a value?
a string, you will need get a void pointer
from esp and then get a char pointer to which Answer: f->eax
points that void pointer. The char pointer will point to the first
element of the string …
Create example
» void SysCall_Create(struct intr_frame *f) {
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char *file_name;
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int file_size;
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printf("create system call\n");
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file_name = (char *)*(int *)(f->esp + 4);
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file_size = *(int *)(f->esp + 8);
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if (filesys_create(file_name, file_size)) f->eax = true;
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else f->eax = false;
» }
» …
» case SYS_CREATE:
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SysCall_Create(f);
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break;
» …
Problem 1: If the pointer above PHYS_BASE,
It points to Kernel memory! UNSAFE!
All the pointers on the variables,
which you get from the user program,
must be validated!
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Memory issues in Pintos
Useful files
» Have a look at:
Kernel VM
Physical Memory
Kernel process
PHYS_BASE
userprog/pagedir.c – implements functions to deal
with page directories
n threads/vaddr.h – contains functions and macros
for working with virtual addresses
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Check if the pointer is below
User process
Page directory
Check if the pointer is in the page directory
If no entry?
a) Kill user
b) Handle page fault
STRUCT THREAD
Shutdown security (safety?)
» static void syscall_handler (struct intr_frame *f) {
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int syscall_name;
You have to implement
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validate_user_ptr(f->esp); “validate_user_ptr(void *)”
function to check if
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syscall_name = *(int *)(f->esp);
the pointer is
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switch(syscall_name) {
in the user space and
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case SYS_HALT:
in the page directory!
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printf("halt system call\n");
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power_off();
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break;
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default:
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printf("unknown syscall %d\n", syscall_name);
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}
» }
Create security
Other system calls
» void SysCall_Create(struct intr_frame *f) {
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char *file_name;
A string can span across
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int file_size;
several pages in memory!
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printf("create system call\n");
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validate_user_ptr(f->esp + 4)
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» }
» Don’t forget validation of system call
parameters!
validate_user_ptr_str((void *)*(int *)(f->esp + 4))
» Validate buffers for read and write
file_name = (char *)*(int *)(f->esp + 4);
validate_user_ptr(f->esp + 8)
file_size = *(int *)(f->esp + 8);
if (filesys_create(file_name, file_size)) f->eax = true;
else f->eax = false;
system calls!
» File IDs of one process must not be
accessible by other processes
All pointers of the string have
to be valid. You should provide
an efficient solution to validate
as less string pointers as possible!
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Exercises!
Exit(SUCCESS);
» Solve problems step by step
» Discuss each sub-problem
» Discuss different solutions
» Answer questions
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